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Layercake RDFLib with ORDF

Layercake is a branch of rdflib used at the Cleveland Clinic
with FuXi. Amongst other things it has some support for remote
SPARQL stores (relevant to `ticket 105`_). Some additional
information is available from the `wiki page explaining layercake`_
and the `layercake message on FuXi discuss`_.

.. _ticket 105:
.. _wiki page explaining layercake:
.. _layercake message on FuXi discuss:

Installation recipe

Before following the usual install procedure, do::

    pip install pyparsing
    pip install -e svn+

And then for example,::

    pip install hg+
    pip install -e hg+
    pip instlal -e hg+